Processes directed to the production of reaction mixtures comprising substantial amounts of aldehydes and at times lesser amounts of alcohols by the reaction of olefinic compounds with carbon monoxide and hydrogen at elevated temperatures and pressures in the presence of certain catalysts are well known in the art. The aldehydes and alcohols produced generally correspond to the compounds obtained by the addition of a carbonyl or carbinol group to an olefinically unsaturated carbon atom in the starting material with simultaneous saturation of the olefin bond. Isomerization of the olefin bond may take place to varying degrees under certain conditions with the consequent variation in the products obtained. Such processes are generally known in the industry under varying names such as the oxo process or reaction, oxonation and/or hydroformylation.
One disadvantage of prior art hydroformylation processes is their dependence upon the use of catalysts such as cobalt octacarbonyl which require exceptionally high operative pressures to maintain such catalysts in their stable form. Another disadvantage is the difficulty in obtaining hydroformylation products which have a relatively high normal to branched-chain isomer ratio.
U.S. Pat. No. 2,880,241 (Hughes) discloses that rhodium-containing catalysts are effective for oxygenating olefins, thereby permitting a "more selective and a different kind of carbonylation." The patentee states that normally, the cobalt catalyzed oxo reaction produces a mixture of two or more isomers "whereas rhodium catalyzed carbonylation at low temperatures, such as 60.degree.-120.degree. C., permits the production of predominantly (75% to 90%) branched isomer" (see column 1, lines 44-57 of U.S. Pat. No. 2,880,241).
U.S. Pat. No. 3,239,566 (Slaugh et al) discloses that complexes of rhodium and phosphines catalyze the oxo reaction to give oxygenated products predominating in aldehydes and/or alcohols. Slaugh et al prefer to employ trialkylphosphine-rhodium-carbonyl complex catalysts at a temperature of from about 150.degree. C. to about 210.degree. C.. The patentees' examples disclose that their rhodium-containing catalyzed process results in oxygenated products having an isomer distribution of normal aldehydes to branched-chain aldehydes which is comparable to cobalt-containing catalyzed oxo processes.
U.S. Pat. No. 3,527,809 (Pruett et al), the entire contents and disclosure of which is incorporated herein by reference, discloses an oxo process for preparing oxygenated or oxonated products containing a high ratio of normal to branched-chain aldehydes by reacting olefins with carbon monoxide and hydrogen in the presence of certain rhodium hydroformylation catalysts.
Tubular reactors have been employed in carrying out the oxo process; however, it has been reported that the use of such tubular reactors enhances the production of alcohols at the expense of aldehydes. Moreover, the tubular reactors have been employed in conjunction with cobalt catalysts and the employment of high temperatures and pressures. See U.S. Pat. No. 3,113,974 (Heimsch et al).
It is an object of the present invention to provide an improved oxo process for the hydroformylation of olefins with carbon monoxide and hydrogen utilizing rhodium catalysts and mild reaction conditions in a tubular reactor, whereby not only is the predominant product an aldehyde, but the ratio of normal to branched-chain aldehydes is enhanced, a result heretofore believed to be unachievable using a tubular reactor.